The invention concerns lubricating compositions which impart antiwear and anti-scuffing properties with reduced levels of phosphorus. Another aspect of this invention is the lowering of sulfur and/or phosphorus, or the complete elimination of phosphorus, in lubricating compositions intended for lubricants where high amounts of sulfur and/or phosphorous are not desirable.
The trend in recent years in lubricant technology, and specifically in passenger car motor oils, is to reduce the levels of phosphorus in the oil that comes from the antiwear additive called zinc dialkyldithiophosphate (ZDDP). The current levels of phosphorus in motor oils is set at 0.10% P and a movement is underway to reduce this to either 0.08% or 0.05% P, with the eventual elimination of phosphorus altogether. The problem is maintaining adequate antiwear protection in the oil at a reasonable cost. The concern with P in motor oil is its poisoning effect on catalytic converters. Likewise, there is a movement toward reducing the overall presence of sulfur in motor oils, both because of environmental concerns, as well as because of the effect of sulfur as a corrosive. As sulfur based compounds are now commonly used as antiwear additives, there is a strong desire to reduce the amount of these compounds needed to achieve effective antiwear protection.
It is known that certain borate ester composition possess antifriction properties as well as other desirable lubricating characteristics as disclosed in U.S. Pat. No. 4,389,322, which is hereby incorporated by reference.
U.S. Pat. No. 5,641,731 and U.S. patent application Publication No. 2003/0119682 teach a 7-component lubricant additive, comprising the following components: an oil soluble molybdenum additive, zinc dithiophosphate, non-aqueous PTFE, a poly-alpha-olefin, a diester, a viscosity index improver and a borate ester composition. The non-sulfur Molyvan® 855 organo molybdenum amide complex is tested as a specific Mo component, and Mo dithiocarbamate is also indicated as a possible additive. The reference relates to a comprehensive formulation seeking to improve numerous properties simultaneously, of which antiwear protection is only one. While the patentee reports improvements in antiwear properties, the presence of zinc dithiophosphate is at very high levels. Thus, the dispersant inhibitor containing compound which includes zinc dithiophosphate has a phosphorus component of roughly 1 mass %. As the reference teaches adding the dispersant inhibitor at levels of about 11 vol % (about 12.3 mass %), the P level in the lubricant would be about 0.1 mass %. Thus, this high P level renders this formulation unsuitable for the new GF-4 requirements.
Surprisingly, it has been discovered that organo borate ester composition produce a synergistic antiwear effect in combination with certain organic sulfur, organic phosphorus and non-sulfur molybdenum compounds, with the result that lower amounts of these compounds may be used while retaining or increasing their effectiveness in the performance level of the lubricant. Excellent improvements in the performance of known antiwear additives can be achieved by using small amounts of a borate ester composition having low concentrations of boron in combination with these additives. The additives which show a synergistic effect in combination with borate ester composition include dithiophosphates such as zinc dialkyl dithiophosphate (ZDDP), dithiocarbamates such as molybdenum dithiocarbamates and ashless dithiocarbamate, thiadiazoles and non-sulfur molybdenum amide complexes such as Molyvan® 855 lubricant additive. It is surprising that tenacious films are being formed on metal surfaces when the combined additive is used in a lubricant, and that these films enhance the performance of all the different classes of antiwear compounds listed above.
With respect to dithiophosphate compounds, this is advantageous in that the amount of phosphorus may be greatly lowered, to well below 0.05 mass %, while retaining the necessary performance. Further, it is also advantagous to be able to lower the total sulfur used in antiwear additives, as new GF-4 specifications will limit the allowable sulfur. The two-component system combinations discovered by the applicants provide excellent performance, with a lower amount of the sulfur compounds (and lower phosphorus in the case of dithiophosphates), thereby permitting a lower sulfur (and/or phosphorus) total in the overall lubricant. As for non-sulfur molybdenum compounds such as the molybdenum amide complex Molyvan® 855 additive, cost of antiwear protection can be reduced by using lower amounts of the additive in combination with the organo borate ester composition.